Umm instead of making the functions affect everyone globaly why not just loop through each player and target them indivisualy to see if they have the add on, and if not allow a forwarded function that tells the plugin that the client does not have it and let the author decide what to do from there. ^^

It doesn't effect people globally. If you use a function, IE, it will send a message to the client to interact with cl_metamod, it either has to succeed, meaning the client will have to have cl_metamod, or the function will fail.